The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
The disclosed technology relates to communication systems, and more particularly, to estimating a channel quality indicator (CQI) for multiple input multiple output (MIMO) systems that use transmit diversity schemes.
In a data transmission system, it is desirable for information, often grouped into packets, to be accurately received at a destination. A transmitter at or near the source sends the information provided by the source via a signal or signal vector. A receiver at or near the destination processes the signal sent by the transmitter. The medium, or media, between the transmitter and receiver, through which the information is sent, may corrupt the signal such that the receiver is unable to correctly reconstruct the transmitted information. Therefore, given a transmission medium, sufficient reliability is obtained through careful design of the transmitter and/or receiver, and of their respective components.
However, the transmitter may be unaware of how the channel will affect a transmitted signal, and may not be able to transmit information in a way that will be effective for a particular channel. For example, the transmitter may be a wireless router, where the channel varies depending on its surroundings. One technique to increase reliability when the transmitter does not have information about the channel is to use a CQI scheme to communicate channel conditions from the receiver to the transmitter. For example, a transmitter may transmit reference or pilot signals to the receiver. The receiver may process the received pilot signals to generate channel feedback information corresponding to the current conditions of the transmission channel. This feedback computed by the receiver may be referred to as Channel Quality Indicator (CQI) and may be based on any suitable channel parameter such as Signal-to-Interference and Noise Ratio (SINR), Signal-to-Noise Ratio (SNR), Signal-to-Noise plus Distortion Ratio (SNDR), power of received signal, supportable data rates, supportable modulation and/or coding rates, and supportable throughput. The receiver may then transmit the CQI information to the transmitter, e.g., in the form of a CQI message. The transmitter may use the CQI information to determine suitable transmission parameters and/or schemes, such as the number of transmit antennas to activate, the modulation and coding rates, and/or the bandwidth allocation.
One technique to increase reliability in a transmission system is to increase the number of outputs (e.g., transmit antennas) that transmit the same information. Thus, the same information may travel through multiple paths, allowing the receiver or receivers to more reliably estimate the transmitted information. This transmission technique is referred to as transmit diversity, and a system with multiple outputs (e.g., transmit antennas) and multiple inputs (e.g., receiver antennas) is commonly referred to as a multiple input multiple output (MIMO) system.
Conventional CQI methods estimate channel conditions using complete channel state information and are effective when the channel is relatively slowly fading in the time domain and less frequency selective in the frequency domain. However, transmit diversity schemes may present various challenges to estimating CQI using these conventional methods. For example, in some transmit diversity schemes, the channel state information may be limited or inaccurate, and portions of the communication channel may not be available or may be too complex to compute or manipulate. In some transmit diversity schemes, effective channels may experience fast fading in the time domain or large frequency selective fading in the frequency domain, rendering the conventional CQI methods ineffective.